Tight gas reservoirs are natural gas reservoirs in which the matrix has a very low permeability, for example, lower than 0.5 mD (millidarcy), or even lower than 0.1 mD. In these very compact reservoirs, the recovery ratios are about 10% with the production technologies used today. The simple drilling of a well is therefore insufficient to obtain economical gas production.
In order to overcome this problem, one alternative is to increase the exchange surface between the matrix and the wellbore. At the present time, this exchange surface is usually increased using a technique called hydraulic fracturing. This technique consists in hydraulically creating a fracture that is kept open by the installation of support agents. This serves to create a larger exchange surface between the matrix and the wellbore.
However, the hydraulic fracturing technique has several drawbacks:                it is costly to implement and requires specific equipment at the well surface;        during implementation, it is not possible to control the fracturing direction. This is because the fractures tend to advance according to the stress field in the matrix;        finally, the injection of pressurized fluid is liable to damage and pollute the exchange surface that is created.        
Alternative techniques to hydraulic fracturing also exist, for increasing the contact surface between the wellbore and the reservoir as effectively. Techniques for drilling horizontal drains, in particular, have been used for many years (for example, “Horizontal Radial Drilling System” by W. Dickinson and R. W. Dickinson, 1985, Society of Petroleum Engineers). These techniques are applied by means of drilling rigs conventionally used in the industry, comprising a directional bit hydraulically driven by the drilling fluid.
The drawback of this type of borehole, in addition to the large scale surface equipment requirements, stems from the need to circulate a fluid to drive the bit, and this is liable to damage the drain drilled during contact with the formation.